The present invention relates general to nuclear power plants, and more specifically to the emergency systems of such power plants.
A nuclear power plant typically has a nuclear reactor and a reactor coolant system (RCS) for removing heat from the reactor and to generate power. The two most common types of reactors, boiling water reactors (BWRs) and pressurized water reactors (PWRs) are water-based. In a pressurized water reactor (PWR), pressurized, heated water from the reactor coolant system transfers heat to an electricity generator, which includes a secondary coolant stream boiling a coolant to power a turbine. In BWRs, the reactor boils the reactor coolant directly to produce steam for the electricity generator. The RCS section downstream of the electricity generators but upstream of the reactor typically is called the cold leg, and downstream of the reactor and upstream of the electricity generators is typically called the hot leg.
If a failure occurs in the RCS, in what is typically called a loss of coolant accident (LOCA), the nuclear core does not properly cool, temperature begins to rise in the reactor. The temperature of the fuel elements in the core rises and, if not checked, can cause melt and potentially void the reactor, releasing the melt into the containment building. One type of LOCA which can occur in both PWRs and BWRs is a main steam line break.
During a LOCA accident, a standard evolution of pressure and temperature inside the containment involves an increase in pressure to a few bars in 5-18 hours, with a maximum temperature around 150° C., which is reduced to atmospheric pressure and temperature in a few days. Nuclear power plants are designed to weather such an event with a considerable safety margin. The cooling process is based on the physical properties of water and air at those temperatures.
During a LOCA accident, an emergency core cooling system (ECCS) can be activated to cool the reactor by providing additional water to the RCS. An ECCS typically thus includes a high-pressure pump such as a centrifugal charging pump/high pressure injection pump (CCP/HPIP pump) exiting into the RCS. This can pump water from the refueling water storage tank (RWST), such as an in-containment RWST (IRWST), or a containment sump into the cold leg of the RCS. A volume control tank receiving water passing through a heat exchanger from the RCS cold leg can also provide water to the CCP/HPIP pump.
The ECCS also typically has a low-pressure pump, such as a residual heat removal or safety injection system pump (RHR/SIS pump), which can provide water from the RWST or containment sump to the cold and hot legs of the RCS, as well as water to a containment spray system. A heat exchanger is typically provided after the RHR/SIS pump.
The ECCS also typically has accumulators connected to the cold leg of the RCS storing water under pressure using pressurized nitrogen, as well as a pressurizer for providing extra pressure to the hot leg of the RCS and providing expansion volume to accommodate RCS volume and temperature transients.
Post-accident cooling has to do with both phenomena of natural convection heat transfer of air and the vapor phase inside the containment following a LOCA accident as well as with the boiling heat transfer inside the core during the LOCA condition.
The article entitled “In-Vessel Retention Enhancement through the Use of Nanofluids” describes using nanofluids for In-Vessel retention enhancement during an accident scenario. The conceptual nanofluid injection system includes two small tanks of concentrated nanofluid, with each tank capable of supplying enough nanofluid to provide enhancement predicted by a computational model. The injection is considered to occur upon the manual actuation of valves connected to injections lines. Instructions to actuate these valves are required to be placed in the severe accident procedures. The injection is said to be driven by gravity and overpressure provided by accumulators attached to the tanks. The injection lines are such that they can terminate in the reactor cavity, in the recirculation lines, or in the IRWST, depending on the physical space limitations within containment.